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Title: Modeling the release of E. coli D21g with transients in water content

item Bradford, Scott
item WANG, YUSONG - University Of Iowa
item TORKZABAN, SAEED - Commonwealth Scientific And Industrial Research Organisation (CSIRO)
item SIMUNEK, JIRI - University Of California

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/7/2015
Publication Date: 5/8/2015
Citation: Bradford, S.A., Wang, Y., Torkzaban, S., Simunek, J. 2015. Modeling the release of E. coli D21g with transients in water content. Water Resources Research. 51:3303-3316. doi: 10.1002/2014WR016566.

Interpretive Summary: Most microbial transport studies have been conducted under steady-state water saturation conditions. Results typically suggest limited mobility of micoorganisms in the subsurface, especially in unsaturated soils that occur near the soil surface. A mathematical model was developed to simulate the transport and release of microorganisms in variably saturated soils. Simulation results provided an excellent description of available experimental data, and demonstrated that episodic pulses of microorganisms will be released and transported with flowing water in unsaturated soils. This provides an increased risk of microbial contamination to surface and groundwater resources. Consequently, the developed model and simulations results will be of interest to scientist, engineers, government regulators, and public health officials who are concerned with microbial contamination of drinking and recreational waters, and fresh produce.

Technical Abstract: Transients in water content are well known to mobilize colloids that are retained in the vadose zone. However, there is no consensus on the proper model formulation to simulate colloid release during drainage and imbibition. We present a model that relates colloid release to changes in the air-water interfacial area (Aaw) with transients in water content. Colloid release from the solid-water interface (SWI) is modeled in two steps. First, a fraction of the colloids on the SWI partitions to the mobile aqueous phase and airwater interface (AWI) when the Aaw increases during drainage. Second, colloids that are retained on the AWI or at the air-water-solid triple line are released during imbibition as the AWI is destroyed. The developed model was used to describe the release of Escherichia coli D21g during cycles of drainage and imbibition under various saturation conditions. Simulations provided a reasonable description of experimental D21g release results. Only two model parameters were optimized to the D21g release data: (i) the cell fraction that was released from the SWI (fr) and (ii) the cell fraction that partitioned from the SWI to the AWI (fawi). Numerical simulations indicated that cell release was proportional to fr and the initial amount of retention on the SWI and AWI. Drainage to a lower water content enhanced cell release, especially during subsequent imbibition, because more bacteria on the SWI were partitioned to the AWI and/or aqueous phase. Imbibition to a larger water content produced greater colloid release because of higher flow rates, and more destruction of the AWI (smaller Aaw). Variation in the value of fawi was found to have a pronounced influence on the amount of cell release in both drainage and imbibition due to changes in the partitioning of cells from the SWI to the aqueous phase and the AWI.